Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is becoming increasingly widespread and their fields of application range from “industrial” and “sports and leisure” to high performance aerospace components.
A common composite material is made up from a laminate of a plurality of fibre layers, interleafed with resin layers. Although the carbon fibres have some electrical conductivity, the presence of the interleaf layers means that this is only exhibited in the plane of the laminate. The electrical conductivity in the direction orthogonal to the surface of the laminate, the so-called z-direction, is low.
This lack of conductivity in the z-direction is generally accepted to contribute to the vulnerability of composite laminates to electromagnetic hazards such as lightning strikes. A lightning strike can cause damage to the composite material which can be quite extensive, and could be catastrophic if occurring on an aircraft structure in flight. This is therefore a particular problem for aerospace structures made from such composite materials.
A well known method of addressing this problem is to include a conductive element, e.g. metallic mesh or foil, at or near the external surface of the composite material.
A common way for such a conductive element to be applied is to deposit manually, by the hand of a skilled handler, the conductive element onto the surface of a mold for an aerospace body structure. Subsequently, the composite material, comprising sheets of structural fibres and thermosetting resin, typically in the form of prepregs, is laid on top of the conductive element. However, the structural layers tend to be laid down in an automated manner by means of a so-called automatic tape lay-up apparatus, or ATL.
A typical ATL machine requires a roll of material which is loaded onto a mandrel and fed via a system of rollers to the ATL head. Typically the route will involve a cutting stage and an optional heating stage. The cutting stage ensures that the dimensions of the material are exactly as required, as any deviation can result in an unacceptable finish. At the head of the ATL there is normally two application methods, the “shoe” of the ATL or the “compaction roller” of the ATL head. Whichever method is employed the material is brought down into contact with the surface and pressure is applied onto an uppermost backing sheet of release paper. The tacky lowermost surface adheres under pressure and the backing sheet is removed automatically. During this process the material is exposed to very high tensions of between 50 and 300 N over a typical product width of 300 mm.
Following the lay-up procedure, the arrangement is cured by exposure to elevated temperature, and optionally to elevated pressure, to produce a cured composite laminate. The cured laminate is taken from the mold and the conductive element is present as part of the structure on the exterior of the aerospace structure it is to form.
Composite materials have successfully been demonstrated to provide strong, lightweight and reliable structures for aerospace applications. There is thus a trend to an increasing use and gradual replacing of the traditional metallic structure. As a result, ever greater areas of aircraft are being produced from composite material. Consequently, the labour required to lay down the conductive element is becoming an increasing cost and time burden on the manufacture of such structures.
Ideally such conductive electromagnetic hazard protector elements would be laid down automatically, as the composite material is, however this is fraught with difficulty and no satisfactory automated solution is known to exist.